If the clicking noise is heard when you first turn the key, before the starter cranks; it usually comes from either a weak battery or corroded battery terminals. But if the clicking noise is heard just after the motor starts running, it comes from a low engine oil level, or dirty hydraulic valve lifters, or an unsuitable brand or type of oil that "drains down" after the motor is stopped (thus causing a delay in oil pressure build up the next time the motor is started). A clogged oil filter, or the wrong model of oil filter, or oil that is overdue for a change, will also cause such noises.